Download Physics Mind on Physics Modules for Work and Energy (1,2,3,4,5,7

Physics
Mind on Physics Modules for Work and Energy (1,2,3,4,5,7,8) These are worth 10
quiz points each. The other modules are worth 2 bonus quiz points each. You
might want to save Sublevel 2 for later – power is covered later in our chapter.
Chapter 5 Objectives
1. Define the work done by a constant force and work done by a force which varies
with position. (Recognize that the work done by a force can be positive, negative,
or zero; describe at least one example of each case.)
2. Recognize that the gravitational potential energy function, PEg – mgy, can be
positive, negative, or zero, depending on the location of the reference level used
to measure y. Be aware of the fact that although PE depends on the origin of the
coordinate system, the change in potential energy, (PE)f – (PE)I is independent of
the coordinate system used to define PE. Work following problem:
A 755 N diver drops from a board 12.0 m above the water surface. use
conservation of mechanical energy to find his speed 6.00 m above the
water surface.
3. Understand that a force is said to be conservative if the work done by that force
on a body moving between any two points is independent of the path taken.
Nonconservative forces are those for which the work done on a particle moving
between two points depends on the path. Account for nonconservative forces
acting on a system using the work-energy theorem. In this case, the work done by
all nonconservative forces equals the change in total mechanical energy of the
system. Work Example 5.7 and the following exercise on pages 133 and 134.
4. Understand the distinction between kinetic energy, potential energy, and total
mechanical energy of a system. State the law of conservation of mechanical
energy, noting that mechanical energy is conserved only when conservative forces
act on a system. This extremely powerful concept is very important in all areas of
physics.
5. Relate the work done by the net force on an object to the change in kinetic energy.
The relation W = ∆KE = KEf – KEi is called the work-energy theorem and is valid
whether or not the resultant force is constant. that is if we know the net work
done on a particle as it undergoes displacement, we also know the change in its
kinetic energy. This is the most important concept in this chapter, so you must
understand it thoroughly.
Conceptual Questions at end of chapter:
2,4,6,8,10,18
Problems at end of chapter:
5.
11.
14. 90.0 J
21.
23.
27.
33.
37.
45.
51.
5
WORK AND ENERGY
PROBLEMS
*Indicates intermediate level problems.
1.
What is the kinetic energy of a 3000-kg car moving at 55.0 mi/h? How much heat, in joules, will be
lost through friction in the brake linings when the car is brought to a stop?
9.07 x 105J
2.
A very good major league pitcher can throw a baseball at 100 mi/h, while an average pitcher can
throw the ball at a speed of about 80.0 mi/h. If a baseball has a mass of 0.150 kg, find the kinetic
energy of the ball in each case.
Fast pitch: 150 J; slower pitch: 96.0 J
3.
A gas molecule in the room you are sitting in may have a speed of about 50.0 m/s. If its mass is
equal to 5.00 × 10–26 kg, what is its kinetic energy?
6.25 x 10-21- J
4.
A 1200-kg vehicle is moving along a horizontal surface with a speed of 20.0 m/s. What work must
be done by the brakes to bring the vehicle to rest in 20.0 s?
2.40 x 105 J
5.
What is the gravitational potential energy relative to the ground of a 0.150-kg baseball at the top of a
100-m tall building?
147 J
6.
A baseball player decides to show his skill by catching a 0.150-kg baseball dropped from the top of a
100-m tall building. What is the speed of the baseball just before it strikes his glove?
44.3 m/s
7.
A 70.0-kg high-jumper leaves the ground with a speed of 6.00 m/s. How high can he leap?
1.84 m
8.
A baseball player throws a baseball straight up into the air with an initial speed of 20.0 m/s. Find (a)
the maximum height to which the ball rises, and (b) the speed of the ball when it is halfway up to its
maximum height.
a. 20.4 m; b. 14.1 nm/s
9.
A woman weighing 500 N glides across some ice, starting her glide with a speed of 4.00 m/s. If the
coefficient of friction between the skates and the ice is 0.115, how far does she go before coming to
rest? Solve this problem by using the method of work-kinetic energy.
7.10 m Be sure to use the correct method
10. What average power is developed by an 800-N professor while running at constant speed up a flight
of stairs rising 6.0 m if he takes 8.0 s to complete the climb?
0.80 hp
11.A motor is used to pull an 80.0-kg skier along a horizontal surface at a constant speed of 2.00 m/s to
enable him to learn to keep his balance. If the coefficient of kinetic friction between the skis and the
surface is 0.150, what horsepower motor is required?
0.315 hp
12. A machine lifts a 300-kg crate at constant speed through a height of 5.00 m in 8.00 s. Calculate the
power output of the machine.
2.46 hp
13. You have just found the "Lost Dutchman" gold mine, 80.0 ft below ground level. However, the mine
is flooded, and you discover that in order to keep it dry enough to mine, you must pump water out at
the rate of 60.0 lb every second. What is the minimum horsepower motor that can be used to perform
this task?
8.73 hp
14. A speedboat requires 130 hp to move at a constant speed of 15.0 m/s ( 33 mi/h). Calculate the
resistive force due to the water at that speed.
6. 47 x 103 N
15. Find the height from which you would have to drop this textbook so that it would have a speed of 10
m/s just before it hits the pavement.
5.1 m
16. A driver of a 7.50 × 103 N car passes a sign stating "Bridge Out 30 Meters Ahead." She slams on the
brakes, coming to a stop in 10.0 s. How much work must be done by the brakes on the car if it is to
stop just in time? Neglect the weight of the driver, and assume that the negative acceleration of the car
caused by the braking is constant.
-1.38x105J
17. A 5.00-g bullet moving at 600 m/s penetrates a tree trunk to a depth of 4.00 cm. (a) Use work and
energy considerations to find the average frictional force that stops the bullet. (b) Assuming that the
frictional force is constant, determine how much time elapses between the moment the bullet enters
the tree and the moment it stops moving.
0.133 ms